A comparatively new development that has become known among gasoline engine combustion methods is the HCCI (Homogeneous Charge Compression Ignition) method, which is also referred to as the CAI (Controlled Auto Ignition) method. That method is distinguished by having a significant potential to save fuel compared with conventional spark-ignition operation.
CAI engines operate with a homogeneously (uniformly) distributed, lean (λ>1) mixture of fuel and air. Ignition is initiated in this case by the rising temperature as compression takes place and by any free radicals and intermediates or precursors of the preceding combustion process that have remained in the combustion chamber. Unlike the case of a conventional gasoline engine, this auto-ignition is completely desirable and forms the basis of the principle of why a spark plug is not needed in CAI operation. Outside a given part-load range, a spark plug is needed.
In CAI operation, the charge composition is ideally so uniform that combustion begins simultaneously throughout the combustion chamber. To produce stable CAI operation, internal or external exhaust gas recirculation or exhaust gas retention may be employed. By exhaust gas recirculation/retention it is to a certain extent possible to monitor the combustion position.
CAI combustion produces a comparatively low combustion temperature with very homogeneous mixture formation, which leads to a large number of exothermic centers in the combustion chamber and therefore to a combustion process that proceeds very evenly and rapidly. Pollutants such as NOx and soot particles may accordingly be avoided almost completely in comparison with stratified operation. It is therefore possible where appropriate to dispense with expensive exhaust gas treatment systems such as NOx storage catalysts. At the same time, efficiency is increased in comparison with spark-ignited combustion.
CAI engines are as a rule equipped with direct gasoline injection and a variable valve train, a distinction being made between fully variable and partially variable valve trains. An example of a fully variable valve train is EHVC (electro-hydraulic valve control) and an example of a partially variable valve train is a camshaft-controlled valve train with 2-point lift and phase adjuster.
In CAI engines, regulation of dynamic engine operation is a great challenge. The expression “dynamic engine operation” is used herein to mean on the one hand changing of the type of operation between the auto-ignition operating mode (CAI mode) and the spark-ignition operating mode (SI mode), and on the other hand also load changes within the CAI mode. Changes to the operating point in dynamic engine operation should take place as steadily as possible in respect of torque and noise, which, however, proves difficult on account of the factors described hereinafter:
In CAI operation, there is no direct trigger in the form of spark-ignition to initiate combustion. Accordingly, the combustion position has to be ensured by very carefully coordinated control of the injection and air system at every cycle of a dynamic changeover.
A further difficulty arises on changing between SI operation and CAI operation: in SI operation, the residual gas compatibility is comparatively low and therefore as little residual gas as possible should be retained in the cylinder. In contrast, however, CAI operation requires precisely a comparatively large proportion of residual gas. It is therefore not possible for the proportion of residual gas to be gradually raised “in preparation”, as it were, before a change from SI operation to CAI operation, and conversely, when changing from CAI operation to SI operation, the proportion of residual gas may not already be lowered in advance since this would lead to considerable disturbance of the combustion behavior to the point of misfiring.
The effect described above also means that, at a changeover from SI operation to CAI operation under the control of a conventional linear controller, too much residual gas and/or residual gas that is too hot is generally retained for the first CAI cycles. Accordingly, combustion that is too early, that is, too loud to the point of knocking, and potentially damaging to the engine is obtained. That in turn means that the change in type of operation entails troublesome noise development.
Similar phenomena also occur at load changes within CAI operation. At an abrupt change from a lower to a higher load point, too little residual gas and/or residual gas that is too cold is retained in the first cycle following the load change, which leads to combustion that is too late (compared with the desired value) to the point of misfiring. In the reverse case of an abrupt change from a higher to a lower load value, combustion occurs by contrast too early and too loudly.
Consequently, both an abrupt change in load and a changeover between CAI operation and SI operation at the same load brings with it the problem of a rapid change in the combustion noise, which the driver generally finds disturbing.
There is therefore a need for an improved method for operating internal combustion engines, especially internal combustion engines that are operable, at least in a part-load range, in an operating mode with auto-ignition, which method involves a less disturbing variation of the combustion noise.